Billing change has forced me to seek new doctors on short notice. Provider and insurance company are blaming each other. Thoughts on or experiences with this?

Long version:
2 weeks ago, we got a letter from a provider that we use informing us that a due to a change in billing processes, they would start billing their services as outpatient hospital as opposed to office visit.

This will result in a huge increase in our out-of-pocket costs. We confirmed with billing people there that services for which we were paying a $20 co-pay earlier in the year will cost us hundreds of dollars.

Frustrated by having to change doctors we've been seeing for years on less than 2 weeks notice, I've been trying to find out the who, what, when and why of the changes.

The provider insists that my insurance company changed their billing requirements, and that it wasn't finalize until mid-July, so they couldn't have told me earlier. The insurances company tells me that there was no such change on their side.

I've pursued several channels on both sides. Somebody is either completely wrong or flat out lying, and I'm determined to find out who.

Has anyone run into a situation like this before? Any suggestions for getting to the bottom of this?

I have not heard of this before. I have heard of physicians changing which insurance companies they will work with, and this can happen at any time. But changing to outpatient hospital for a simple office visit is not acceptable. It would only be appropriate if your treatment were an injection or some type of in office surgery, even minor.